1. Field of the Invention
The present invention relates to a projective transformation convergence calculation method for observing a flat part (planar region) of an object in images captured by two cameras by using a projective transformation between the captured images.
2. Description of the Related Art
If there is an object having a flat part in images captured by two imaging cameras and one captured image (reference image) is transformed by an appropriate projective transformation matrix, it is possible to match an image of the flat part of the object in the transformed image with an image of the flat part of the object in the other captured image (standard image). In this case, this type of projective transformation matrix capable of image transformation depends on the spatial posture (for example, a normal line direction) and position (for example, a distance from the optical center of the camera) of the plane including the flat part of the object. In other words, the projective transformation matrix defines the spatial position and posture of the plane including the flat part of the object in the captured images.
As a technique for observing the flat part of the object in two captured images (measuring the posture or the like of the flat part) using the above, there is conventionally known a technique disclosed in, for example, a document titled “Extraction of Planar Region and Obstacle Detection Using Stereo Images” (Akihito Seki and Masatoshi Okutomi, IPSJ Transactions on Computer Vision and Image Media, Vol. 45, No. SIG13 (CVIM10), December 2004) (Nonpatent Document 1). In this technique, a plurality of appropriate templates are prepared for the projective transformation matrix between the images captured by the two vehicle-mounted cameras in order to extract a road plane on which the vehicle is running and the projective transformation matrix capable of the above image transformation is estimated by the convergence calculation with the templates used as the initial values of the projective transformation matrix. Preparing the plurality of templates as the initial values of the projective transformation matrix is, in other words, preparing a plurality of initial values of parameters representing the plane defined by the projective transformation matrix.
The technique disclosed in Document 1 is intended to increase the robustness of the estimation of the projective transformation matrix for the plane to be measured by preparing the plurality of initial values of the projective transformation matrix in the convergence calculation or initial values of the plane to be measured and intended to prevent the dispersion of the estimation or to prevent an inappropriate projective transformation matrix from being estimated (a plane different from an actual plane from being estimated).
The technique disclosed in Nonpatent Document 1, however, is based on a premise that an error in position between the plane defined by the initial values and the actual plane to be measured (an error in distance from the optical center of the camera) is sufficiently small. Therefore, if the error in position between them is relatively large, there is a possibility that the required number of iterative calculations is excessively increased until the projective transformation matrix is well converged or that it is impossible to converge the projective transformation matrix.
Furthermore, the necessity of performing the convergence calculation for the plurality of initial values causes a problem that a computational load is easily excessive until the projective transformation matrix is conclusively determined.